1. Field of the Invention
The present invention relates generally to voltage regulator circuits and, more particularly, to regulator circuits for supplying a regulated and adjustable output voltage.
2. Description of the Prior Art
U.S. Pat. No. 3,617,859 discloses a so-called "bandgap" voltage reference circuit recently adopted in many semiconductor integrated circuit voltage regulators. The circuit employs three cascaded, matched transistors formed in a common substrate and utilizes the temperature dependent characteristics of a transistor emitter-base voltage to achieve a regulator circuit exhibiting an essentially zero temperature coefficient. The three transistors are coupled in a manner such that the negative temperature coefficient of the emitter-base voltage of one transistor is compensated or offset by the positive temperature coefficient of the emitter-base differential voltage between the remaining two transistors which are operated at greatly different current levels.
In the aforementioned patent, the zero temperature coefficient, bandgap voltage reference circuit is connected across the inverting and noninverting input terminals of an operational amplifier having an output terminal adapted to supply a regulated output voltage. The source of unregulated voltage is connected to a supply input terminal of the operational amplifier and to one terminal of the bandgap voltage reference circuit through a current source. While this voltage regulator circuit arrangement is satisfactory for many applications, it exhibits a number of drawbacks. For example, the circuit is suitable for only a very limited change in load and will supply only a very low value of regulated voltage slightly above the theoretical semiconductor energy bandgap voltage (e.g., about 1.5 volts for silicon transistors). Thus, the designation as a "bandgap" voltage regulator. To supply higher regulated voltages, several circuit units must be "stacked" to arrive at multiples of 1.5 volts. Moreover, separate active current supplying devices are required for the bandgap voltage reference circuit and for the operational amplifier. In addition, the circuit exhibits a relatively high standby power consumption. This is particularly undesirable in applications where the source of unregulated voltage applied to the regulator circuit is derived from a battery. For example, portable, battery powered electrochemical measuring systems, for measuring ion concentration such as solution pH, require a stable regulated reference voltage during measurement. Obviously, if the battery is unnecessarily drained during standby intervals, the usefulness of such measuring systems in the field is compromised.
As a result, a need exists for a voltage regulator circuit exhibiting the advantages of the prior circuit, such as inherent temperature stability, without the standby current drain and other disadvantages. The present invention meets these needs.